Optical spectroscopy can be used to determine characteristics and concentrations of constituents in samples. See, e g., Skoog, D. A. and J. J. Leary, Principles of Instrumental Analysis, Fort Worth: Saunders, 1992. In some applications, however, changing measurement conditions, such as instrument and environment changes, can lead to undesirable errors. Accordingly, there is a need for devices and methods that can reduce such errors, for example in the quantitative optical spectroscopic measurement of attributes or analytes in bodily tissue, blood or other biological samples. Such devices and methods can be especially useful when the spectral absorbance of the attribute or analyte is small relative to the total absorbance of the sample.